The present invention relates generally to an electrophotographic image forming apparatus. More particularly, the invention relates to an electrophotographic image forming apparatus which is suitable for forming a color image.
An electrophotographic image forming apparatus operates to obtain image data to print from a personal computer and to print the image data on a printing medium, such as a sheet of paper. More particularly, the electrophotographic image forming apparatus develops the image data into bitmap information and then turns a laser ON and OFF on the basis of the bitmap information for irradiating a laser beam onto a photosensitive body, such as a photosensitive drum, photosensitive film and the like. By irradiation of the laser beam, an electrostatic latent image is formed on the photosensitive body. The electrostatic latent image is developed into a toner image to be transferred onto the printing medium, such as paper, to form a printed image. The image formed on the printing medium is fixed by heating.
When a color image is formed on the printing medium, the image data from the personal computer or the like is developed into individual bitmap data per color. Then, the latent image corresponding to each color is formed and developed at successive times. For instance, at first, a latent image corresponding to magenta image is formed and developed, and then, a latent image corresponding to a yellow image is formed.
As set forth above, the electrophotographic image forming apparatus having a color printing capability is provided with a plurality of developer units. If a particular developer unit is operated continuously for a time in excess of a given period, a density fluctuation, such as a lateral stripe, may be caused in an output image. For example, such a problem is caused when color printing is performed after a large amount of a printing of monochrome image is performed.
A study for the cause of the problem set forth above has been made by the applicants. As a result, it has been found that, while the developer unit for one color corresponding to a single color mode is in operation, the developer rollers of other developers are continuously opposed to the charged belt surfaces so that toner depositing on the surface of the developer roller is subject to the influence of the electrical charge of the charged belt. As a result, the toner is firmly bound with the roller so as to interfere with the formation of a uniform charged toner layer on the peripheral surface of the developer roller. This can be a cause of fluctuation in the developed image.
The present invention provides an electrophotographic image forming apparatus which can restrict or eliminate the occurrence of a density fluctuation, such as lateral stripe and, thereby, can enhance the image quality.
According to the first aspect of the present invention, an electrophotographic image forming apparatus comprises: a photosensitive body; exposure means for forming an electrostatic latent image on the photosensitive body; at least two developer means for developing the electrostatic latent image in mutually distinct colors for respectively forming toner images of mutually distinct colors; transfer means for transferring the toner images of mutually distinct colors on a printing medium in an overlapping manner; fixing means for fixing the toner images transferred onto the printing medium; and when one of the developer means is maintained inoperative, that developer means is forcedly operated in a reset state.
In the preferred construction, the electrophotographic image forming apparatus may further comprise an intermediate transfer body to which is transferred a plurality of toner images, and an image formed on the intermediate transfer body is then transferred to the printing medium. One of the developer means may be held inoperative during a monochrome printing sequence. The color painting sequence may be performed after forcedly operating the one of the developer means in reset state.
In the alternative, the forced operation of the one of developer means may involve rotation of a developer roller therein.
According to the second aspect of the present invention, an electrophotographic image forming apparatus comprises: a photosensitive body; exposure means for forming an electrostatic latent image on the photosensitive body; at least two developer means for developing the electrostatic latent image in mutually distinct colors for respectively forming toner images of mutually distinct colors; transfer means for transferring the toner images of mutually distinct colors on a printing medium in an overlapping manner; fixing means for fixing the toner images transferred onto the printing medium; counting means for counting the number of printed pages; judgment means for making a judgment as to whether one of the developer means has been held inoperative during printing of a predetermined number of pages; and the one of the developer means is operated on the basis of a judgment made by the judgment means.
According to the third aspect of the present invention, a color electrophotographic printing apparatus comprises: a photosensitive body to be exposed by image data per each color for forming an electrostatic latent image thereon; means for developing the electrostatic latent image of each color on the photosensitive body with a corresponding color of toner; an intermediate transfer body receiving toner images of respective colors on the photosensitive body in an overlapping manner for forming a multi-color toner image; means for transferring the multi-color image formed on the intermediate transfer body onto a printing medium; means for fixing the multi-color toner image on the printing medium by thermal fixing; the color electrophotographic printing apparatus having a single color printing sequence for performing outputting of a single color image by operating only developing means corresponding to a color of the input image data when only image data of one color is input, the color electrophotographic printing apparatus also having a developer means resetting sequence operative after printing in the single color printing sequence for a large number of times, if image data requiring operation of the developer means which is held inoperative during the single color printing sequence, is input, for returning a surface condition of a developer roller in the developer means to a normal state continuously subject to influence of an electrical charge on the surface of the photosensitive body during execution of the single color printing sequence, before initiating a printing operation with operation of the developer means which is held inoperative during the single color printing sequence.
Preferably, when the single color printing sequence is repeated for a number of times greater than or equal to a predetermined, the developer means which is held inoperative during the single color printing sequence is operated regularly for preventing the surface of the developer roller from becoming abnormal and, thereby, for returning the surface condition of the developer roller to a normal state.
According to the fourth aspect of the present invention, an electrophotographic image forming apparatus comprises: printer unit including latent image forming means, latent image developing means provided for each printing color for developing the latent image to form toner image corresponding to the latent image, toner image transferring means for transferring, the toner image onto a printing medium, and image fixing means for fixing the toner image on the printing medium, the printing unit being operative at least in a single color printing mode where only one of the image developing means is operated and the remaining image developing means are held inoperative and a multi-color printing mode where a plurality of image developing means are operated; monitoring means for monitoring the condition of the image developing means during the single color printing mode of operation of the printer unit for detecting an abnormal state of the image developing means; and normalizing means responsive to the monitoring means detecting an abnormal state of the image developing means for performing a normalizing process for normalizing the image developing means in an abnormal state before initiation of operation in the multi-color printing mode operation.
The monitoring means may count up the number of printed pages in the single color printing mode for detecting an abnormal state of the image developing means when a counter value becomes greater than or equal to a predetermined criterion. The normalizing means may be responsive to the monitoring means detecting the abnormal state of the image developing means for performing an initialization process.
According to the fifth aspect of the present invention, an electrophotographic image forming apparatus comprises: a printer unit including latent image forming means, latent image developing means provided for each printing color for developing the latent image to form a toner image corresponding to the latent image, toner image transferring means for transferring the toner image onto a printing medium, and image fixing means for fixing the toner image on the printing medium, the printing unit being operative at least in a single color printing mode where only one of the image developing means is operated and the remaining image developing means are held inoperative and a multi-color printing mode where a plurality of image developing means are operated; and normalizing means operative at every predetermined number of pages of print in the single color printing mode for performing a normalizing process for maintaining the image developing means in a normal state.
The normalizing means may operate the image transferring means for causing transfer of an image on the image developing means held in the inoperative state during the single color printing mode the operation.